Field
The disclosed concept pertains generally to circuit interrupters and, more particularly, to a system and method for wide range monitoring of current in a configurable circuit interrupter employing an electronic trip units.
Background Information
Electrical switching apparatus such as circuit interrupters and, in particular, circuit breakers (e.g., of the molded case variety), are well known in the art. See, for example, U.S. Pat. No. 5,341,191.
Circuit breakers are used to protect electrical circuitry from damage due to an overcurrent condition, such as an overload condition or a relatively high level short circuit or fault condition. Molded case circuit breakers typically include a pair of separable contacts per phase. The separable contacts may be operated either manually by way of a handle disposed on the outside of the case or automatically in response to an overcurrent condition. Typically, such circuit breakers include: (i) an operating mechanism which is designed to rapidly open and close the separable contacts, and (ii) a trip unit which senses overcurrent conditions in an automatic mode of operation. Upon sensing an overcurrent condition, the trip unit sets the operating mechanism to a trip state, which moves the separable contacts to their open position.
Industrial molded case circuit breakers often use a circuit breaker frame which houses a trip unit. See, for example, U.S. Pat. Nos. 5,910,760; and 6,144,271. The trip unit may be modular and may be replaced in order to alter the electrical properties of the circuit breaker.
It is well known to employ trip units which utilize a microprocessor to detect various types of overcurrent trip conditions and to provide various protection functions, such as, for example, a long delay trip, a short delay trip, an instantaneous trip, and/or a ground fault trip.
Circuit breakers must operate and provide both circuit protection and metering functionality over a wide range of currents. The full range of the currents for protection and metering can cover a range of 100,000:1. As an example, a circuit breaker measures currents from 1 ampere to 100,000 amperes for both metering and protection purposes. Currents below the rated current of the circuit breaker are important to users from a metering perspective, and it is desired to report to the end-user the level of these currents to a high degree of precision. Currents above the rated current of the circuit breaker must still be monitored, although not with the same precision as metered currents that are below the rated current of the circuit breaker.
Traditionally, current transformers have been used to provide the current sensing function in circuit breakers. However, the magnetic material present in current transformers can limit the range of operation because of magnetic saturation effects. By eliminating the magnetic material (e.g., by replacing it with a substantially lower permeability material, or by introducing a significant air gap), a sensor may be created that operates linearly over a much wider range. Such sensors tend to have a low impedance voltage output that is proportional to the rate of change of the primary current. Because of this response, these devices are often called di/dt sensors, or in special cases Rogowski coils after their inventor.
To compute the primary current from a di/dt sensor, its output must be integrated. The nature of the integration process is that the result depends on the input (in this case the di/dt sensor output) for all previous time. This fact presents a problem in trip units. In particular, all trip units must be powered by current. Thus, power to an electronic device that is able to perform this integration to get the primary current measurement is not initially available at startup. This results in errors that can shift protection functions in circuit breakers outside of their specified range.
One solution to this problem is to perform integration passively with just a resistor and capacitor. No power is required in this scheme and the proper choice of the resistor and capacitor will produce a voltage signal on the capacitor that is proportional to the primary current. However, this scheme has a number of drawbacks. For example, to meet the required precision over the wide range of currents for both protection and metering, the resistor and capacitor must have low drift with respect to temperature and time. This can result in requiring large, expensive resistors or capacitors. Also, the signal on the capacitor is reduced significantly in amplitude below that of the di/dt sensor's output, which results in the need for complicated processing of low current signals.